Kono Sankaku Love Kome wa Shiawase ni Naru Gime ga Aru.

The unprecedented, most precious love-triangle romantic comedy in history, is about to begin! In the new school year, Tenma Yashiro, an ordinary high school boy, is in the same class with two most beautiful girls in school, Reira Tsubaki and Rinka Kou. Rira is happy and innocent, while Rinka is cool and stand-offish. Tenma wants to keep his distance from these two overwhelmingly beautiful girls and lead a peaceful school life, but one day, he discovers Rinka's forbidden secret, and his life changes drastically...? (Source: Comic Alive, translated)